How Come Until Now? (なぜ今まで, Naze Ima Made) is chapter 600 of the Naruto manga.
Shocked by this revelation, Kakashi and Guy started asking the once masked man whether he truly was Obito, but upon seeing his Mangekyō Sharingan, Kakashi noted that there was now no doubt about it. However, Obito reiterated his point that it was insignificant what they called him, and all that mattered to him was his Eye of the Moon Plan. Kakashi had a flashback to the time when Obito died, and then asked whether he had survived the cave in. This led Naruto to ask them who the person in front of them was. Guy answered that he was a former classmate of theirs who supposedly died during the Third Shinobi World War. Kakashi, still in shock, asked Obito that if he had survived then why had he not made this fact known until now. Obito retorts that whether he survived or not was unimportant. However, if Kakashi really had to have an answer to why Obito had done what he did, it was simply because Kakashi had let Rin die. Even more devastated by this response, Kakashi asked him if he was going to blame him for it. Obito answered that he did not see any point in blaming this "useless" reality which would "disappear" soon enough. Naruto tried to rally his sensei, telling him that right now, Obito's plan needed to be stopped and that he would listen to the story later. Obito used the Fire Release: Blast Wave Wild Dance technique sending a giant vortex of flames towards his opponents. Kakashi, unable to register what was happening, simply stood there despite Guy calling out to him. Naruto, however, intercepted the attacks with Kurama's tails. Before any side could act further, they were interrupted by Madara's appearance on their battlefield. Standing beside the younger Uchiha, the legendary shinobi remarked that Obito seemed to be having fun, much to the onlookers' surprise.
In the flashback sequence, Kakashi is shown receiving the cut in his face, which also cuts his mask. In the next page, still in the flashback, the cut in his mask is gone.